What's in a for?

class: center, middle

# Advanced Scala
## What's in a for?
Neville Li

@sinisa_lyh

Sep 2016

---

# Map and flatMap

```scala
trait List[A] {
  map(f: A => B): List[B]
  flatMap(f: A => GenTraversableOnce[B]): List[B]
}
```

```scala
trait Option[A] {
  map(f: A => B): Option[B]
  flatMap(f: A => Option[B]): Option[B]
}
```

```scala
trait Future[A] {
  map(f: A => B): Future[B]
  flatMap(f: A => Future[B]): Future[B]
}
```

---

# Handling options

```scala
def plus(x: Option[Int], y: Option[Int]): Option[Int] =
  if (x.isDefined && y.isDefined) Some(x.get + y.get) else None
```

```scala
def plus(x: Option[Int], y: Option[Int], z: Option[Int]): Option[Int] =
  if (x.isDefined && y.isDefined && z.isDefined) Some(x.get + y.get + z.get) else None
```

### Ugh...

```scala
def plus(x: Option[Int], y: Option[Int]): Option[Int] =
  x.flatMap(a => y.map(b => a + b))
```

```scala
def plus(x: Option[Int], y: Option[Int], z: Option[Int]): Option[Int] =
  x.flatMap(a => y.flatMap(b => z.map(c => a + b + c)))
```

### Not much better...

---

# For to the rescue

```scala
for(x <- c1; y <- c2; z <-c3) {...}
```
is translated to

```scala
c1.foreach(x => c2.foreach(y => c3.foreach(z => {...})))
```

- - -

```scala
for(x <- c1; y <- c2; z <-c3) yield {...}
```
is translated to

```scala
c1.flatMap(x => c2.flatMap(y => c3.map(z => {...})))
```

- - -

[http://docs.scala-lang.org/tutorials/FAQ/yield.html]()

---

# List comprehensions

```scala
scala> for (x <- List("a", "b", "c"); y <- List(1, 2, 3)) yield (x + y)
res0: List[String] = List(a1, a2, a3, b1, b2, b3, c1, c2, c3)
```

equals
```scala
List("a", "b", "c").flatMap(x => List(1, 2, 3).map(y => x + y))
```

- - -

```scala
def plus(a: Option[Int], b: Option[Int], c: Option[Int]): Option[Int] =
  for (x <- a; y <- b; z <- c) yield (x + y + z)
```

equals
```scala
a.flatMap(x => b.flatMap(y => (c.map(z => x + y + z))))
```

---

# More list comprehensions

```scala
val f1: Future[Response1] = backend1.request(payload)
val f2: Future[Response2] = backend2.request(payload)
def compute(r1: Response1, r2: Response2): Response = /* ... */

val response: Future[Reponse] =
  for {
    r1 <- f1
    r2 <- f2
  } yield (compute(r1, r2))
```
Because `Future[T]` has `map` and `flatMap`.

- - -

```scala
val g: Gen[(Double, Double)] =
  for {
    x <- Gen.choose(-1.0, 1.0)
    y <- Gen.choose(-1.0, 1.0)
  } yield (x, y)
```
Because `Gen[T]` has `map` and `flatMap`.

---

class: center, middle
# The End
## Happy Yielding